Merge pull request #15885 from opensourcerouting/gcc-unrecognized-ms-…

…anon-tag

build: get rid of "unrecognized … anon-tag"

configure.ac

@@ -275,7 +275,16 @@ AC_DEFUN([AC_C_FLAG], [{
     AC_CACHE_CHECK([[whether $CC supports $1]], cachename, [
 	AC_LANG_PUSH([C])
 	ac_c_flag_save="$CFLAGS"
-	CFLAGS="$CFLAGS $1"
+	dnl GCC ignores unknown -Wno-whatever flags, but errors on -Wwhatever
+	dnl except when it ignores them it prints:
+	dnl   cc1: note: unrecognized command-line option ‘-Wno-whatever’ may have been intended to silence earlier diagnostics
+	dnl which is annoying as hell.  So check for the positive flag instead.
+	flag_add="$1"
+	if test "$flag_add" != "${flag_add#-Wno-}"; then
+	    CFLAGS="$CFLAGS -W${flag_add#-Wno-}"
+	else
+	    CFLAGS="$CFLAGS $flag_add"
+	fi
 	AC_COMPILE_IFELSE(
 		[AC_LANG_PROGRAM([[$4]])],
 		[